mm: parallelize clear_gigantic_page

hulk inclusion
category: feature
bugzilla: 13228
CVE: NA
---------------------------

Parallelize clear_gigantic_page, which zeroes any page size larger than
8M (e.g. 1G on x86).

Performance results (the default number of threads is 4; higher thread
counts shown for context only):

Machine:  Intel(R) Xeon(R) CPU E7-8895 v3 @ 2.60GHz, 288 CPUs, 1T memory
Test:     Clear a range of gigantic pages (triggered via fallocate)

nthread   speedup   size (GiB)   min time (s)   stdev
      1                    100          41.13    0.03
      2     2.03x          100          20.26    0.14
      4     4.28x          100           9.62    0.09
      8     8.39x          100           4.90    0.05
     16    10.44x          100           3.94    0.03

      1                    200          89.68    0.35
      2     2.21x          200          40.64    0.18
      4     4.64x          200          19.33    0.32
      8     8.99x          200           9.98    0.04
     16    11.27x          200           7.96    0.04

      1                    400         188.20    1.57
      2     2.30x          400          81.84    0.09
      4     4.63x          400          40.62    0.26
      8     8.92x          400          21.09    0.50
     16    11.78x          400          15.97    0.25

      1                    800         434.91    1.81
      2     2.54x          800         170.97    1.46
      4     4.98x          800          87.38    1.91
      8    10.15x          800          42.86    2.59
     16    12.99x          800          33.48    0.83

The speedups are mostly due to the fact that more threads can use more
memory bandwidth.  The loop we're stressing on the x86 chip in this test
is clear_page_erms, which tops out at a bandwidth of 2550 MiB/s with one
thread.  We get the same bandwidth per thread for 2, 4, or 8 threads,
but at 16 threads the per-thread bandwidth drops to 1420 MiB/s.

However, the performance also improves over a single thread because of
the ktask threads' NUMA awareness (ktask migrates worker threads to the
node local to the work being done).  This becomes a bigger factor as the
amount of pages to zero grows to include memory from multiple nodes, so
that speedups increase as the size increases.

Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Signed-off-by: Hongbo Yao <yaohongbo@huawei.com>
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Tested-by: Hongbo Yao <yaohongbo@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>